Recent simulations suggest that trend line decision rules applied to curriculum-based measurement of reading progress monitoring data may lead to inaccurate interpretations unless data are collected for upward of 3 months. The authors of those studies did not manipulate goal line slope or account for a student's level of initial performance when evaluating the accuracy of progress monitoring decisions. We explored how long progress needs to be monitored before ineffective interventions can be accurately identified using actual data. We calculated classification accuracy statistics to evaluate the extent to which recommendations from three common and two novel decision rules correctly predicted spring performance across six levels of duration (8, 10, . . . 18 weeks) and two goal types (normative and default spring benchmark). Comparing the median of the last three observations as well as current trend with expected performance at a given week consistently yielded higher positive agreement rates than data point or prediction-based decision rules. Decision rule performance improved as duration increased, but a point of diminishing returns was observed. Decisions based on normative goals yielded consistently higher chance-corrected agreement outcomes.